Use build.zig for runtime builds

[bvisness]

The Zig build system seems like it will be nice for a few reasons - remove our need for MSVC on Windows, get caching, etc. Most of the groundwork for this has already been laid by Andrew and Loris anyway - however, we're not planning to do this until Zig 0.12 releases since it seems like build stuff is in flux (and not documented).